This invention relates generally to electric submersible pumps and more particularly concerns systems for supplying oil to such pumps during downhole operation.
The upper operating temperature for an electric submersible pumping system is limited by the degradation of the motor oil and the motor insulation due to temperature and by the ability of the thrust bearing to function at elevated temperatures. It has long been recognized that a continuous fresh oil supply increases the life expectancy of the unit. Present oil feed systems depend on the use of complex, surface controlled, pressure balancing and metering systems and require specially developed sub-surface pressure regulating valves that operate at elevated temperatures in the well fluid. These complex regulating systems are used because of the difference in the height of the column of fluid in the well bore and the height of the column in the oil supply tubing, which could amount to several thousand feet. The motor and the seal system could not handle the pressure that this fluid level differential represents. Even if they could, the volume of oil required if oil was allowed to flow freely would be cost prohibitive. In normal operation, the electric submersible pump motor oil is contained in a closed system. The expansion of the motor oil when the motor comes up to operating temperature is offset by storage in a reservoir and is returned to the system when the motor is shut down. On the initial start, the excess oil is vented to the well bore. Such a system typically requires a quarter inch inner diameter tube extending some three to six thousand feet downhole to the motor. In a free flow condition, several barrels of oil per day flow to the motor. This translates into thousands of dollars a day just for the oil necessary to keep the system running.
A further problem with the present fresh oil supply systems is that the oil is introduced at a bellows below the motor with the oil then passing upwardly through the motor and being released at a relief port at the top of the motor. This reduces the effectiveness of the oil by the time it reaches the pothead cavity which is the most likely breakdown point in the system.
It is, therefore, an object of this invention to provide an oil supply system for an electric submersible pump which eliminates the need for a downhole pressure regulator. It is a further object of this invention to provide an oil supply system for an electric submersible pump in which the continuous flow of oil is limited to less than ten and preferably to approximately one to five gallons of oil per day. Another object of this invention is to provide an oil supply system for an electric submersible pump which does not require a downhole oil reservoir. It is also an object of this invention to provide an oil supply system for an electric submersible pump in which the oil is introduced to the pump motor at the pothead cavity.